MDEV-34515: Reduce context switching in purge

Before this patch, the InnoDB purge coordinator task submitted
innodb_purge_threads-1 tasks even if there was not sufficient amount
of work for all of them. For example, if there are undo log records
only for 1 table, only 1 task can be employed, and that task had better
be the purge coordinator.

srv_purge_worker_task_low(): Split from purge_worker_callback().

trx_purge_attach_undo_recs(): Remove the parameter n_purge_threads,
and add the parameter n_work_items, to keep track of the amount of
work.

trx_purge(): Launch purge worker tasks only if necessary. The work of
one thread will be executed by this purge coordinator thread.

que_fork_scheduler_round_robin(): Merged to trx_purge().

Thanks to Vladislav Vaintroub for supplying a prototype of this.

Reviewed by: Debarun Banerjee

storage/innobase/include/que0que.h

@@ -209,17 +209,6 @@ que_eval_sql(
 	const char*	sql,	/*!< in: SQL string */
 	trx_t*		trx);	/*!< in: trx */
 
-/**********************************************************************//**
-Round robin scheduler.
-@return a query thread of the graph moved to QUE_THR_RUNNING state, or
-NULL; the query thread should be executed by que_run_threads by the
-caller */
-que_thr_t*
-que_fork_scheduler_round_robin(
-/*===========================*/
-	que_fork_t*	fork,		/*!< in: a query fork */
-	que_thr_t*	thr);		/*!< in: current pos */
-
 /** Query thread states */
 enum que_thr_state_t {
 	/** in selects this means that the thread is at the end of its

storage/innobase/include/srv0srv.h

@@ -624,6 +624,15 @@ Complete the shutdown tasks such as background DROP TABLE,
 and optionally change buffer merge (on innodb_fast_shutdown=0). */
 void srv_shutdown(bool ibuf_merge);
 
+/**
+ Fetches and executes tasks from the purge work queue,
+ until this queue is empty.
+ This is main part of purge worker task, but also
+ executed in coordinator.
+ @note needs current_thd to be set beforehand.
+*/
+void srv_purge_worker_task_low();
+
 } /* extern "C" */
 
 #ifdef UNIV_DEBUG

storage/innobase/que/que0que.cc

@@ -166,40 +166,6 @@ que_thr_init_command(
 	thr->state = QUE_THR_RUNNING;
 }
 
-/**********************************************************************//**
-Round robin scheduler.
-@return a query thread of the graph moved to QUE_THR_RUNNING state, or
-NULL; the query thread should be executed by que_run_threads by the
-caller */
-que_thr_t*
-que_fork_scheduler_round_robin(
-/*===========================*/
-	que_fork_t*	fork,		/*!< in: a query fork */
-	que_thr_t*	thr)		/*!< in: current pos */
-{
-	fork->trx->mutex_lock();
-
-	/* If no current, start first available. */
-	if (thr == NULL) {
-		thr = UT_LIST_GET_FIRST(fork->thrs);
-	} else {
-		thr = UT_LIST_GET_NEXT(thrs, thr);
-	}
-
-	if (thr) {
-
-		fork->state = QUE_FORK_ACTIVE;
-
-		fork->last_sel_node = NULL;
-		ut_ad(thr->state == QUE_THR_COMPLETED);
-		que_thr_init_command(thr);
-	}
-
-	fork->trx->mutex_unlock();
-
-	return(thr);
-}
-
 /**********************************************************************//**
 Starts execution of a command in a query fork. Picks a query thread which
 is not in the QUE_THR_RUNNING state and moves it to that state. If none

storage/innobase/srv/srv0srv.cc

@@ -1556,7 +1556,6 @@ static bool srv_purge_should_exit(size_t old_history_size)
 
 /*********************************************************************//**
 Fetch and execute a task from the work queue.
-@param [in,out]	slot	purge worker thread slot
 @return true if a task was executed */
 static bool srv_task_execute()
 {
@@ -1696,6 +1695,13 @@ static void release_thd(THD *thd, void *ctx)
 	set_current_thd(0);
 }
 
+void srv_purge_worker_task_low()
+{
+  ut_ad(current_thd);
+  while (srv_task_execute())
+    ut_ad(purge_sys.running());
+}
+
 static void purge_worker_callback(void*)
 {
   ut_ad(!current_thd);
@@ -1703,8 +1709,7 @@ static void purge_worker_callback(void*)
   ut_ad(srv_force_recovery < SRV_FORCE_NO_BACKGROUND);
   void *ctx;
   THD *thd= acquire_thd(&ctx);
-  while (srv_task_execute())
-    ut_ad(purge_sys.running());
+  srv_purge_worker_task_low();
   release_thd(thd,ctx);
 }